(i) Field of the Invention
The present invention relates to a molded body comprising at least 65 vol % of magnetic powder, particularly to a technique for increasing fixing strength between a magnetic mold material and an external electrode.
(ii) Description of the Related Art
Technological innovations in reducing the sizes and increasing the speeds of electronics in recent years are remarkable. Along with them, a reduction in the sizes and an improvement in the performances of electronic parts such as a mold coil have been desired. To reduce the size and improve the performance of the mold coil, magnetic materials showing high performance such as high magnetic permeability and excellent moldability have been desired, in particular.
Heretofore, magnetic mold materials having excellent magnetic properties as exemplified by high magnetic permeability have been studied. Japanese Patent Application Laid-Open No. 304018/1993 discloses a magnetic mold material having magnetic properties improved by comprising at least 60 vol % of amorphous alloy powder.
When an external electrode is integrally-molded into a molded body used for the mold coil or the like, the external electrode is directly brought into contact with and fixed to a magnetic mold material without use of an adhesive. The magnetic mold material primarily comprises magnetic powder and a resin. The magnetic powder has no adhesion, and the resin has adhesion. The surface of the external electrode makes contact with both the resin having adhesion and the magnetic powder having no adhesion.
In the case of conventional, common magnetic mold materials, the proportion of the resin is as high as at least 50 vol %, and the surface of the external electrode can contact the resin to a sufficient extent. Accordingly, high fixing strength can be attained even if the external electrode is integrally-molded. However, if the proportion of the magnetic powder in the magnetic mold material is increased to improve magnetic properties, the proportion of the resin is inevitably decreased. When the proportion of the resin in the magnetic mold material is decreased, the resin cannot make contact with the surface of the external electrode to a satisfactory extent. As a result, the fixing strength of the external electrode deteriorates, and the external electrode may fall off.
TABLE 1CompoundingRatio [wt %]Magnetic PowderFixingMagneticVolume FillingStrengthSamplePowderResinRate [vol %][kgf]18515492.22928660.8
TABLE 1 shows the fixing strengths of the external electrodes of molded bodies obtained by use of magnetic mold materials differing in the filling rate of magnetic power. Hereinafter, samples 1 and 2 shown in TABLE 1 will be described. The external electrodes of the samples 1 and 2 were prepared by use of a rolled phosphor bronze plate. The rolled phosphor bronze plate has been commonly used as a material for the external electrode. Further, the samples 1 and 2 use magnetic mold materials prepared by blending silicon steel (Fe—Si based) powder having a maximum particle size of 45 μm with a novolac epoxy resin in the compounding ratios shown in TABLE 1. The external electrodes and the magnetic mold materials were press-molded to obtain molded bodies each having 2.8 mm on a side. The molded bodies of the samples 1 and 2 are formed in the same manner as the molded body of the present invention to be described later is formed and have the same external configuration as that of the molded body of the present invention. Further, measurements of the fixing strengths of the samples 1 and 2 were made in the same manner as described in a suitable embodiment of the present invention. Details thereof will become apparent by referring to the suitable embodiment of the present invention.
As is clear from TABLE 1, the sample 1 having a magnetic powder filling rate of about 50 vol % has a fixing strength of 2.2 kgf. In the case of an electronic part having about 3 mm on a side, it must retain an external electrode fixing strength of generally at least 12 N (1.22 kgf), preferably at least 15 N (1.53 kgf). Since the fixing strength of the sample 1 is 2.2 kgf (21.6 N), the sample 1 has a sufficient external electrode fixing strength of at least 15 N.
Meanwhile, in the sample 2, the magnetic powder filling rate was set to be at least 65 vol % so as to improve magnetic properties. The fixing strength of the sample 2 is 0.8 kgf, which is significantly lower than the sample 1. This fixing strength is lower than 12N which is a reference value. A highly reliable electronic part cannot be obtained even if an electronic part such as a mold coil is prepared by use of the sample 2.
An object of the present invention is to provide a molded body that retains sufficient fixing strength between a magnetic mold material and an external electrode and has high magnetic properties.